As well known, tufted fabrics are those fabrics in which a plurality of pile yarns are pushed or stitched through a primary backing or substrate forming loops which comprise the fabric surface or which loops may be cut to form a cut pile fabric surface. Machinery for forming these tufted fabrics is likewise well known. In machinery of this type, one or more needle bars having a plurality of needles threaded with individual yarns are reciprocated, typically vertically, to pass the needles through the substrate to form loops which can remain as the fabric surface or be subsequently cut to form cut pile. The yarns are fed to the needle bars from yarn supply creels by one or more feed rolls. Where straight stitches are formed in the warp direction and the needle bar or bars are not shifted in the weft direction, the yarn feed rolls are typically controlled to provide a constant yarn feed to the needles. With a weftwise shift in the one or more of the needle bars, the yarn feed rolls are controlled to provide more or less yarn to the needles so that a smooth face of constant pile height relative to the substrate is maintained on the fabric surface.
In certain tufting machines, the feed rolls which control the yarn feed to the needles of the needle bar or bars are driven by servomotors which allow different lengths of yarn to be fed to the needles upon shifting the needle bar or bars. That is, when a needle bar shifts so that the needles are aligned with different hooks or loopers, there is insufficient yarn fed to the needles to preclude a chop or low line from appearing across the face of the fabric. To preclude this and to provide a smooth face across the fabric surface, a variation in yarn feed is adjusted by adjusting the servomotors driving the feed rolls to compensate for the extra yarn required to accommodate the weftwise movement of the needle bar. Thus, to avoid robbing previous tufts or stitches of yarn due to insufficient yarn feed to the next tuft or stitch, the servomotor controlled feed rolls in the past have been designed, programmed and utilized to provide the yarn feed compensation necessary to create a smooth face in the resulting fabric. That is to say, feed rolls for controlling yarn feed to the tufting needles have heretofore been driven by servomotors to allow different or preselected lengths of yarn to be fed to the needles when the needle bar or bars are shifted to new hook or looper positions, to enable the resulting fabric surface to remain smooth and level.
Further, tufted fabrics, i.e., fabrics having tufts of different heights throughout the fabric, have been provided in the past, for example, in carpets. Various techniques have been previously employed to provide tufted piles of such different heights. For example, cam disks have been used for varying the height of individual tufts in a stitch row in the weft direction. As the cam disks rotate, the yarn feed tension changes and differences in pile height are thus created. Roll pattern attachments, pattern slats and control scrolls have similarly been used to vary pattern height. However, in none of these prior tufting arrangements, has precise and accurate control of the height of each tuft been achieved in such manner that the difference in height between next-adjacent tufts in one or more stitch rows in the warp direction can be greater than 3/32 inch (2.38 mm). That is to say, with prior mechanisms, the variation in tuft height from one tuft to a succeeding tuft in the same stitch row in the warp direction has not exceeded 3/32 inch (2.38 mm). Where a jump in height of next-adjacent tufts in a warpwise stitch row in excess of 3/32 inch (2.38 mm) was required, the resulting fabric necessarily, because of the type of tufting apparatus used, had tufts of an intermediate height intervening between the tufts of the desired heights. That is, the incremental height adjustment of warpwise immediately next-adjacent tufts was limited in prior machines to 3/32 inch (2.38 mm) or less in the fabric, hence limiting the nature of the pattern in the fabric. These intermediate tufts produced an undesirable tapering effect in the tufts, albeit the fabric was patterned with warpwise non-next-adjacent tufts ultimately having a height differential in excess of 3/32 inch (2.38 mm).